The present invention relates to a method of forming a light-weight, fiber-reinforced thermoplastic resin product, and a light-weight molded product. In particular, the invention relates to a method comprising injection molding of specific fiber-reinforced thermoplastic resin pellets or of specific fiber-reinforced thermoplastic resin pellets and a small amount of a foaming agent for forming a light-weight, fiber-reinforced, particularly glass fiber-reinforced, thermoplastic resin product with good surface condition, high strength and high stiffness, and also to a light-weight molded product.
As having high tensile strength, high stiffness and high heat resistance, glass fiber-reinforced resins are of great use as various materials. In particular, injection-molded products of resins of the type are the most popular because of their good moldability. However, they are defective in that their specific gravity increases with the increase in the glass fiber content and that their strength is lowered since glass fibers are often cut during molding. In addition, as being anisotropic, they are further defective in that their warping deformation is great. In order to overcome these defects, proposed was a method of using glass fiber-reinforced resin pellets in which the reinforcing glass fibers have the same length as each pellet to be from 2 to 100 mm and are oriented in parallel with each other (see, for example, JP-B Sho-63-37694, JP-A Hei-3-188131). Also proposed was a method of molding fiber-reinforced thermoplastic resin pellets with fibers having a length of from 10 to 100 mm and oriented in parallel with each other, to produce fiber-reinforced thermoplastic resin moldings with fibers entangled and having a length of from 5 to 100 mm (see JP-A Hei-6-198753). However, these methods are still problematic in that the molded products are heavy-weight, depending on the glass fiber content.
On the other hand, to reduce the weight of glass fiber-reinforced resin moldings, known is a foaming injection molding method in which is used a foaming agent (see JP-A Hei-7-247679). However, this method is problematic in that it requires a large amount of a foaming agent, that it is not easy to attain a foaming magnification of from 2 to 5 times, and that the glass fiber content is naturally limited. Also known is an injection-press molding method comprising injecting a foaming agent-containing resin into the cavity of an opened split mold followed by closing the mold. However, this method is still problematic in that, if a large amount of a foaming agent is used in order to obtain foamed moldings having a high foaming magnification, failed moldings having silver marks of vapor paths in their surface are formed, and that most of the moldings produced have poor strength and stiffness. For these reasons, at present, the known methods could not be put into practical use.
Another method of forming fiber-reinforced resin moldings is known, which comprises plasticizing a mixture of reinforcing fibers and a thermoplastic resin followed by molding it under compression, and which is characterized in that the compression for the surface of the molding in which the reinforcing fibers are intended to be restored expanded is partly released thereby raising the fibers existing in the surface layer of the molding, and/or forming voids partly inside the molding to make the molding have a sound-absorbing region (see WO96/25280). However, this molding method is to produce moldings having a specific, raised surface condition, and could not produce moldings with smooth surface. In addition, as comprising the compression molding step, this method is problematic in that its producibility is low, that the raw material mixture could not be highly homogenized, and that, if the mixture is highly homogenized, the reinforcing glass fibers are much broken. For those reasons, the use of the moldings produced by this method is limited.
We, the present inventors have assiduously studied in order to obtain light-weight, high-strength, fiber-reinforced thermoplastic resin products through injection molding, and, as a result, have found that the intended light-weight products are obtained by a combination of a specific molding material and a specific molding mode, and that the products obtained have good appearance with good surface characteristics of smooth skin. On the basis of these findings, we have completed the present invention.
The invention is to provide a light-weight, fiber-reinforced resin product, which has a skin layer with no void in its surface and of which the inside is highly uniform and has fiber-entangled, continuous open voids with no large hollow, and the product has a high relative bending strength. Specifically, the invention provides the following:
(1) A method of forming a light-weight, fiber-reinforced thermoplastic resin product, comprising;
melting and kneading a molding material, which comprises fiber-containing thermoplastic resin pellets (A) having a fiber content of from 20 to 80% by weight, the fibers being oriented in parallel with each other and having a length of from 2 to 100 mm, or comprises a mixture of the fiber-containing thermoplastic resin pellets (A) and a thermoplastic resin except (A), the fiber content of the mixture being from 5 to 80% by weight;
then injecting the melted resin into the cavity of a mold as so closed that the volume of its cavity is smaller than that of the final molded product; and
before or after the resin injection is completed, opening the mold until the volume of its cavity is equal to that of the final molded product.
(2) The method of forming a light-weight, fiber-reinforced thermoplastic resin product of (1), wherein the fibers are glass fibers and the fiber content is from 20 to 80% by weight.
(3) The method of forming a light-weight, fiber-reinforced thermoplastic resin product of (1), wherein the fibers are organic fibers and the fiber content is from 5 to 80% by weight.
(4) The method of forming a light-weight, fiber-reinforced thermoplastic resin product of (1), wherein the fibers are carbon fibers and the fiber content is from 5 to 80% by weight.
(5) The method of forming a light-weight, fiber-reinforced thermoplastic resin product of (1), wherein a foaming agent in an amount of from 0.01 to 5 parts by weight, relative to 100 parts by weight of the molding material, is added to the molding material.
(6) The method of forming a light-weight, fiber-reinforced thermoplastic resin product of any one of (1) to (5), wherein the thermoplastic resin is a polyolefin-based resin optionally containing a polyolefin as modified with an unsaturated carboxylic acid or its derivative.
(7) The method of forming a light-weight, fiber-reinforced thermoplastic resin product of (1), wherein, in the step of opening the mold until the volume of its cavity is equal to that of the final molded product before or after the resin injection is completed, the part of the mold cavity except its part for forming the edges of the product is opened.
(8) The method of forming a light-weight, fiber-reinforced thermoplastic resin product of (1) , wherein, in the step of injecting the melted resin into the cavity of a mold as so closed that the volume of its cavity is smaller than that of the final molded product followed by the step of opening the mold until the volume of its cavity is equal to that of the final molded product before or after the resin injection is completed, the resin injection is so attained that the volume of the resin injected is smaller than that of the initial mold cavity, then the mobile part of the mold is once pushed toward the injected resin before and after the resin injection is completed to thereby make the cavity completely filled with the injected resin, and thereafter the mold is opened until the volume of the mold cavity is equal to that of the final molded product.
(9) A molded, light-weight, fiber-reinforced thermoplastic resin product having a fiber content of from 20 to 80% by weight and a porosity of from 10 to 80%, and having a skin layer with no void on its surface, the fibers existing in the product having a weight-average fiber length of from 1 to 20 mm.
(10) The molded, light-weight, fiber-reinforced thermoplastic resin product of (9), which has a relative bending strength of not smaller than 80 MPa.
(11) The molded, light-weight, fiber-reinforced thermoplastic resin product of (9) or (10), which is for any of car parts, electric appliances for household use, furniture, and building materials.